This is a synthetic medicinal chemistry proposal to design and develop new pepstatin-derived inhibitors of aspartic proteinases, a class of enzyme that includes rennin, pepsin and cathepsin D. The proposal focuses on four major objectives: 1) synthesize new ketone analogs of pepstatin designed to bind to penicillopepsin and other aspartic proteinases so that x-ray crystal studies of analogs of the tetrahedral intermediate for amide hydrolysis can be carried out. This objective will also include synthesis of statine-containing and hydroxyethylene-containing inhibitors, and their corresponding ketone analogs, derived both from pepstatin and from good enzyme substrates; 2) extension of structure-activity studies for statine and hydroxyethylene isosteres to R. chinensis pepsin, cathepsin D and other aspartic proteinases; 3) continue our C13 NMR studies of the binding of ketomethylene and statone analogues and pepsin to extend the NMR studies to the binding to penicillopepsin. The transformation of the trigonal carbonyl to the tetrahedral gem diol will be monitored by NMR using our reported methods, and this transformation will be correlated with systematic changes in the structure of the peptide, especially those changes in substrate structure known to prevent hydrolysis of the peptide bond by aspartic proteinases; 4) synthesize novel cyclic inhibitor structures based on the x-ray crystal structure of Iva-Val-Val-Sta-OEt-bound to penicillopepsin. The objective here will be to explore new strategies for preparing protease inhibitors with the long range aim of developing small, potentially orally active inhibitors.